In gas metal arc welding with melting, continuously fed electrode, frequently named MIG/MAG-welding, the workpiece is heated primarily by the arc. The electrode is heated, partly by the power supplied when the weld current flows through the electrode stick out i.e. the free electrode end between the contact tip, where the current transfer to the electrode takes place, and the arc, partly by the arc itself. The basic control of the welding process aims at achieving an electrode melting speed which corresponds to the electrode feed speed. Further objects of the control may for instance be to influence the amount of heat transferred to the workpiece.
MIG/MAG-welding takes place in one of three states. In short arc welding, the material transport from the electrode to the workpiece takes place through large short-circuiting droplets. Since the process consists in alternating arc and short-circuiting droplet transitions, the average voltage between the electrode and the workpiece becomes low and thus the heat transfer to the base material will remain moderate. When the supplied power is increased, one passes into the mixed arc area, where the material transport takes place through a mixture of short-circuiting and non-short-circuiting droplets. The result is an unstable arc with significant weld spatter and weld smoke. Welding in this area is normally avoided. At a sufficiently high supplied power, the process enters the spray area, where material transport takes place through small finely dispersed droplets without short circuits. The spatter quantity is clearly lower than in short arc welding. The heat supply to the base material will be greater making this method suitable primarily for thicker workpieces.
The third state is referred to as pulsed welding and means that, by means of advanced control, one controls the proper cut off of the droplets by means of a suitable current pulse. Each pulse cuts off a droplet and the droplets become sufficiently small so as not to short-circuit. This method results in advantages from the spray area in the form of low weld spatter without the disadvantages of large heat transfer. In the following, equipment adapted for MIG/MAG-welding according to the short arc method is considered. The state here alternates between short-circuiting and arc between the weld wire end (electrode end) and the workpiece. The dynamic properties of the weld current source together with the adjustments determine the time of the short-circuiting. During normal welding each short circuit ought to be 0.5-40 milliseconds. Dynamic properties may be created by dimensioning the inner resistance in the weld transformer, inductor and electronic circuits, and the inductance of the inductor. In modern machines, the inductor is frequently of an electronic kind, i.e. a process regulator comprising hardware and software. This is so in order to be able to vary the dynamic properties at the start process in relation to welding during continuance. The dynamic properties of a welding machine determine how fast the welding current can be controlled and adjusted during the welding process. The process regulator thus gives the properties, which influence each individual short-circuiting process by defining, in the process regulator, the current increasing rate during short-circuiting.
The static characteristic of the machine is mainly defined by the inner resistance or its equivalent in a process regulator. Static characteristics of a power supply in a welding machine must be adapted to the welding method that the machine will perform. A MIG/MAG-machine adapted for short arc welding is to be considered as a constant voltage source having a slightly decreasing characteristic, normally 3V per 100 A. This can be compared to a TIG welding machine where instead the current is constant.
In more simple welding machines there is a setting knob for the electrode feed speed and a setting knob for the choice of one of several voltage outlets from the weld transformer in the welding machine. This may be replaced by a wheel for controlling the ignition angle on a thyristor for generating the weld voltage. In modern inverter machines, the weld voltage may be controlled with a great precision. Modern inverter technology with switch mode power supply and micro processor controlled transistors offer faster and more precise control of the static characteristics and the dynamic characteristics compared to other power supply configurations with thyristors or step controlled transformers that need to be adapted for each welding method and welding case.
A well-known problem is that a suitable reference value for the voltage for each electrode speed is dependent on such factors as electrode material, electrode dimension and shielding gas type. A usual manner in welding machines of today is to include in the control computer of the welding machine experience in form of suitable welding parameters for various electrode feed speeds for varying combinations of values of the influencing factors mentioned above, so called synergy lines. Producing such lines for all combinations of influencing factors represents an extensive work in the form of test weldings and documentation. In addition, the material quality may vary between different deliveries and lead to the fact that previously tested synergy lines do not function any longer. Furthermore, shielding gases are now marketed with supplier specific names without specifying the composition of the gas. Also, this leads to problems in having a predetermined quantity of synergy lines suitable for all weld cases. Not even a later repetition of an apparently identical weld case does have to succeed since the composition of the gases or the weld electrode may have been changed by the manufacturer without notice. Obviously, this leads to a troublesome uncertainty when welding a new batch.
Another problem is that the technique up to now does not give a uniform weld result at varying distance between the contact tip in the weld torch and the workpiece, such as during passage of areas of tack welding and corners.